1. Field of the Invention
The invention relates to a process for the injection molding of a plastic mass via an injection element into a cavity with an injection device having an extrusion die connected to the cavity via multiple openings.
2. Description of the Related Art
A prior art injection molding process for producing plastic containers is disclosed in DE 196 17 349 Cl, in which thermoplastic material is pressed into the cavity of an injection molding mold via multiple tubular dies. The plastic material pressed into the injection molding mold through the annular passages of these dies retains the individual layers formed by the annular passages, even in the solidified state and thereafter.
A prior art injection molding extrusion die for the injection of plastics, hardenable resins and rubber is disclosed in EP 0 441 868 B1, in which the position of closing tube in the extrusion die housing is adjustable. Adjustment of the axial position of the closing tube creates a central material flow that is surrounded by a sleeve-shaped material flow supplied separately to the cavity. The material fed into the cavity flows in the same direction as the main axis of the injection molding device.
The object of the invention is to create an injection process and an injecting device that has a simple construction, uses as little energy as possible, and permits the plastic material to have a non-isotropic structure immediately before its entry into the cavity.
The object is attained by a process for injection molding a plastic mass in a cavity using an injection element, including the steps of feeding the plastic mass from the injection element into a cavity through a plurality of material supply openings which forms a plurality of independent flow strands, rotating a portion of the injection element which includes the plural material supply openings, and cutting off the injection by maintaining a rotation of the portion of the injection element having the plural material supply openings after the cavity is filled and cooled.
The object of the invention is also attained by an injection molding unit for injecting a plastic mass into a cavity in a mold, including an injecting device mountable between a machine extrusion die and the mold for receiving the plastic mass from the machine extrusion die and guiding the plastic mass to the cavity of the mold, said injecting device having a material supply part with a mouth portion and a central axis, said mouth portion connectable to the mold and having a plurality of openings through which the plastic mass flows, and a drive operatively connected to said material supply part for rotating said injecting device about said central axis.
According to the invention, the plastic mass is fed into the cavity in a plurality of independent flow threads via the material supply part. The material supply part that produces the flow threads is rotated so that the individual flow threads are turned relative to each other and thoroughly mixed.
The free area of the openings of the material supply part is larger than the diameter of the material supply line in the injection molding machine extrusion die. As a result, there is only a slight pressure build-up in the material supply part during injection into the cavity. For this reason, less total energy is used, specifically, during injection and for the closing force.
Moreover, due to the rotary flow, a lesser quantity of material is required, and the material used is less subjected to shear stress. In addition, the better overall orientation of the plastic material allows products to be produced with significantly thinner wall thicknesses.
After the cavity is filled and cooled, injection is cut off by maintaining the rotation of the material supply openings. As a result, the injection points are equally clean as those known from point connection.
The flow threads may have any desired cross-sectional shape including circular and rectangular, the cross-sections being provided by rings arranged concentrically relative to a central one of the openings.
In an advantageous embodiment, particularly in the case of circular-symmetrical products with small wall thicknesses, e.g., buckets, no confluence points (binding seams) are created, despite the enlarging injection area, because the plastic material injected through the center one of the openings precedes that emerging from the other openings.
The material supply part that feeds the plastic mass to the cavity may comprise many different forms. In one embodiment, this part has a plurality of circular borings that are connected to the central boring of the machine extrusion die.
Although the structure of this material supply part resembles that of the machine extrusion die, the material supply part is also connected to a drive, via which it is placed into rotation. A motor (which, as applicable, is integrated in the mold) drives a pinion, which communicates with the material supply part.
In another embodiment, the material supply part comprises a pot, with a bottom that rests in the fixed mold part and the other end of which is mounted on the machine extrusion die. The entire pot may be placed into rotation by the drive. The inner part of the pot forms a chamber, in which the plastic material is evenly distributed before being injected into the cavity. The openings in the bottom have different diameters. The diameters of the outlying openings are smaller than the central opening and the distance through these outlying openings is longer than that of the central opening.
In another embodiment, the material supply part is constructed in the form of a sleeve which constitutes a connection from the injection molding machine to the mold. In one embodiment, the entire bottom is driven. In another embodiment, one end of the sleeve rests directly on the mold and the bottom is arranged inside the sleeve such that the bottom is drivable through a passage in the side wall.
In a further embodiment, a torpedo-like projection is connected to the bottom of the material supply part. The torpedo-like projection may be placed into rotation from outside the sleeve via an electromagnetic drive.
For especially targeted guidance of the plastic material threads which are fed to the cavity, an impact plate may be provided in the cavity across from the central opening of the material supply part. This impact plate has a crater that guides the plastic material evenly to all sides in the cavity.
The various features of novelty which characterize the invention are pointed out with particularly in the claims annexed to and forming a part of the disclosure. For a better understanding of the invention, its operating advantages, and specific objects attained by its use, reference should be had to the drawing and descriptive matter in which there are illustrated and described preferred embodiments of the invention.